1. Field of the Invention
The invention relates to methods of restructuring bulk-processed fruits and vegetables utilizing drum drying and extrusion, and products made by such methods.
2. Description of the Art
Research on structured fruit products began in the 1940's with a process using alginates for the formation of structured cherries. Droplets, containing cherry puree and alginate, were dropped into a bath of calcium salt to form a skin. Alginate based structured fruits offer the advantage that they are not affected by heat; therefore, these cherries could be used in baked goods (Peschardt, U.S. Pat. No. 2,403,547 1942).
During the 1960's and early 1970's the USDA began to develop structured fruit products. Gelled applesauce, similar to cranberry sauce, was produced in an effort to increase utilization of process-grade apples. Gelled applesauce contained golden delicious apples, sucrose, low-methoxyl pectin, citric acid, calcium lactate and water. The product was marketed mainly as an adjunct to pork, as cranberry is to turkey (Lazar and Morgan, Food Technology March: 52-53, 1964). Several years later, prunes and other dried fruits were gelled using low-methoxyl pectin. These gelled sauces could then be used in bakery goods, cereals, mixes, etc. (Bolin and Nury, Canner/Packer May, 1967). In another study, apricot concentrate was formed into fruit "sheets of cloth" using a drum drier. The cloth was pressed into cubes, artificial halves and bars for use in a variety of food products. Consumers recommended increasing product moisture content and sweetness to improve acceptability (Bolin, Fuller et al., The Bakers Digest March: 30-32, 1973; Bolin, Turnbaugh et al., The Bakers Digest August: 24-25, 1974).
The properties of structured fruits were studied in the late 1970's and early 1980's. These gels simulated fruit texture, but did not contain any fruit. Simulated fruit gels suitable for freeze dehydration were produced. Freeze dried gels were formed using a two step process in which a gelatin containing sodium alginate gel was cooled, sliced and placed in a calcium lactate solution for crosslinkage. When evaluated in yogurt and jello, rehydrated gels scored as well organoleptically as most fresh fruits, with the exceptions of banana and pineapple (Luh, Karel et al., Journal of Food Science 41:89-93, 1976). Later Karel correlated the compressive break strength and fracture energy density of calcium alginate gels with sensory properties. The effects of pectin, gelatin and sucrose on textural properties were also evaluated. The Instron Universal Testing Machine was capable of detecting smaller differences in sample mechanical properties than sensory panelists. Results indicated that pectin improved the sensory acceptability of simulated fruits. The two step method for formation of simulated fruit gels was later improved to increase retention of water soluble components such as ascorbic acid (Luh, Flink et al., Journal of Food Science 42(4):976-981, 1977). A one step procedure using glucono-delta-lactone and dicalcium phosphate dihydrate was developed, eliminating residence in the crosslinking bath (Pelaez and Karel, Journal of Food Processing and Preservation 5:63-81, 1981).
In the 1980's, the three processing methods for the formation of structured fruits from alginates were defined as internal setting, diffusion setting and setting by cooling. Internal setting involves calcium ion release from within the system at room temperature and produces structured fruits with uniform texture. Diffusion setting allows calcium ions to diffuse into the alginate matrix and produces structured fruits with an outer skin and a liquid center. Co-extrusion may also be used to accomplish diffusion setting. The third method involves setting by cooling where gelled ingredients are dissolved in hot water and later set by cooling (Hannigan, Food Engineering March: 48-49, 1983).
Novel expanded snacks were formed in the late 1980's by co-extrusion of rice flour with dried fruits and fruit juice concentrates. A Brabender laboratory extruder was used in this study. Extruded snacks exhibited comparable yield and sensory properties compared to extrudate made exclusively from rice. The juices performed better than the dried fruit in regards to expansion. Citric acid addition did not significantly improve extrudate properties with the exception of improved color retention (Maga and Kim, Lebensm.-Wiss. U.-Technol. 22(4):182-187, 1989).
In the past few years, A. Nussinovitsch has performed several studies on the formation and properties of structured fruits. In the first study, apple pulp and reconstituted grapefruit juice were added to calcium alginate gels at concentrations of 5-96% (Kaletunc, Nussinovitsch et al., Journal of Food Science 55(6):1759-1761, 1990). The mechanical properties of the resultant gels were characterized. This work was the first to incorporate such high concentrations of fruit pulp to gels. In another study, raspberry pulp was added to calcium alginate gels with and without the addition of agar (Nussinovitsch and Peleg, Journal of Food Processing and Preservation 14:267-278, 1990). Agar did not improve product strength, but did increase brittleness and stiffness. Nussinovitsch also investigated the mechanical properties of gel-pulp-sugar composite products formed from various hydrocolloids (agar, carrageenan and alginate), a wide range of sugar concentrations, and various fruit pulps (orange, banana, and apricot) in order to identify means of producing structured fruits with a desired consistency (Nussinovitsch, Kopelman et al., Lebensm-Wiss. U.-Technol. 24:513-517, 1991). Sugar addition strengthened gels up to a maximum point after which gel strength was reduced. Fruit pulp, on the other hand, weakened gel strength down to a minimum point after which the gel system regained strength. Size reduction increased strength. Models were later developed to quantitate the combined effect of fruit pulp, sugar and gum on the mechanical properties of agar and alginate gels (Nussinovitsch, Kopelman et al., supra, 1991).
Recently, two studies concerning structured fruit products were published. Response surface methodology was used to optimize the texture of a sweetened mango pulp-alginate structured fruit product. Structured mango was formed with 90% fruit pulp. This study also developed a method for evaluating the thermostability of textured fruits (Mouquet, Dumas et al., Journal of Food Science 57(6):1395-1400, 1992). A group from Saudi Arabia formed prickly pear sheets from fruit pulp, sucrose, citric acid, sodium metabisulphite and olive oil. These sheets were highly acceptable to a small sensory panel and could be marketed as an alternative to apricot sheets (Ewaidah and Hassan, International Journal of Food Science and Technology 27:353-358, 1992).
In the past, the formation and properties of restructured vegetable products have not been investigated. However, recent research projects have evaluated the formation and properties of restructured sweet potato products. Collins et al., Journal of Food Science 60(3):465-467, (1995) formed sweet potato pieces by stuffing baked sweet potato into cellulose casings and subsequently freezing the product. Troung et al., Journal of Food Science 60(5):1054-1059, (1995) formed restructured sweet potato products in a similar fashion, with the addition of alginate and various salts. Products from both studies exhibited acceptable color, flavor and texture. Kim and Maga, IFT 95 Book of Abstracts 25H-7, (1995) recently produced a snack containing squash and pinto beans.
Most current commercial products and processes utilize only a small percentage of dried fruit, fruit juice concentrate, fruit powder or fruit puree in the final product. The major ingredients are sugars, starches, gels and gums. Molded fruit pieces made with dried plum paste, glycerin, oat fiber and citrus fiber are available commercially. Extruded fruit pieces made from sugar, soybean oil, soy protein, cellulose gum, and natural and artificial colors and flavors are also available commercially. Restructured vegetable products are not available in the current marketplace.